use new {dl,ul}_tbf_as_tbf(_const) helper functions

Change-Id: I223d9d5823c90ff31089420d35a7bf0fe1ebf870
diff --git a/src/pdch_ul_controller.c b/src/pdch_ul_controller.c
index 1cdbb98..df0c500 100644
--- a/src/pdch_ul_controller.c
+++ b/src/pdch_ul_controller.c
@@ -275,7 +275,7 @@
 				item_tbf = item->tbf_poll.poll_tbf;
 				break;
 			case PDCH_ULC_NODE_TBF_USF:
-				item_tbf = (struct gprs_rlcmac_tbf *)item->tbf_usf.ul_tbf;
+				item_tbf = ul_tbf_as_tbf_const(item->tbf_usf.ul_tbf);
 				break;
 			default:
 				OSMO_ASSERT(0);
@@ -316,7 +316,7 @@
 		case PDCH_ULC_NODE_TBF_USF:
 			LOGPDCH(ulc->pdch, DRLCMAC, LOGL_INFO,
 				"Timeout for registered USF (FN=%u): %s\n",
-				item->fn, tbf_name((struct gprs_rlcmac_tbf *)item->tbf_usf.ul_tbf));
+				item->fn, tbf_name(ul_tbf_as_tbf_const(item->tbf_usf.ul_tbf)));
 			tbf_usf_timeout(item->tbf_usf.ul_tbf);
 			break;
 		case PDCH_ULC_NODE_TBF_POLL: